Boleslaw Wrymouth had published his testament dividing his lands among four of his sons before his death in 1138, The "Senioral Principle" established in the testament states that at all times the eldest member of the dynasty is to have supreme power over the rest and is also to control an indivisible "senioral part": a vast strip of land running north-south down the middle of Poland, with Kraków its chief city.

The Senior's prerogatives also include control over Pomerania, a fief of the Holy Roman Empire.

The "senioral principle" will soon be soon broken, leading to a period of nearly two hundred years of Poland's feudal fragmentation.